/*  * 一个简单grep程序，可从文档中提取包含莫些字符串的行  */ public class grep extends Configured  implements Tool{  public static  class grepMap extends Mapper<LongWritable, Text, Text,NullWritable>{   public void map(LongWritable line,Text value,Context context) throws IOException, InterruptedException{    //通过Configuration获取参数    String str = context.getConfiguration().get("grep");    if(value.toString().contains(str)){     context.write(value, NullWritable.get());    }   }  }  @Override  public int run(String[] args) throws Exception {   if(args.length!=3){    System.out.println("ERROR");    System.exit(1);   }   Configuration configuration = getConf();   //传递参数   configuration.set("grep", args[2]);   Job job = new Job(configuration,"grep");   job.setJarByClass(grep.class);   job.setMapperClass(grepMap.class);   job.setNumReduceTasks(0);   job.setMapOutputKeyClass(Text.class);   job.setOutputValueClass(NullWritable.class);   Path in = new Path(args[0]);   Path out = new Path(args[1]);   FileSystem fileSystem = out.getFileSystem(configuration);   if(fileSystem.exists(out))    fileSystem.delete(out, true);   FileInputFormat.addInputPath(job, in);   FileOutputFormat.setOutputPath(job, out);   System.exit(job.waitForCompletion(true)?0:1);   return 0;  }